1. Field of the Invention
This invention relates to a polypropylene resin composition. More particularly, it relates to a polypropylene resin composition which is superior in moldability and provides moulded articles having superior rigidity, resistance to heat distortion, resistance to secondary deformation, resistance to sink mark forming and resistance to warpage deformation when the articles are made therefrom.
2. Prior Arts
With regard to the materials for interior automotive trims, appliance parts, parts of office automation machineries, tendency of utilization of plastics is increasing lately, particularly from the viewpoint of their light weight and economical advantage. Among them, polypropylene resin which is superior in moldability, economical condition, appearance and shape, strength, resistance to climate (weatherability) and durability is desirably used.
However, interior automotive trims or appliance parts which are moulded by using conventionally known polypropylene resins have problem in that they have latent residual stress due to inner strain caused during the process of cooling after moulding. On account of this, when an article in which these parts are used, is exposed to the atmosphere at a temperature of 70.degree.-120.degree. C. at the time of practical utilization, release of internal stress occurs, resulting in secondary deformation of the articles to which these parts are attached and losing value as article of commerce. In order to overcome this problem, polypropylene to which an inorganic filler is incorporated, a so-called inorganic-filler-containing polypropylene resin, is abundantly used. However, if such an inorganic filler-containing polypropylene resin is used, the secondary deformation of moulded articles may be improved but drawbacks are accompanied: The surfaces of moulded articles become liable to be injured by foreign materials and moreover, the injuries whiten and surface appearance is spoiled. Further impact strength is also lowered.
We, the inventors of this invention have made strenuous effort in study to improve the drawbacks of the above-mentioned polypropylene resin composition i.e. the problem of liability of causing secondary deformation. As the result, to be astonishing enough, we have found that
a composition formed by blending a predetermined amount of organic fibers with a propylene-ethylene block copolymer having a 3-16% by weight of ethylene content based on the entire copolymer (hereinafter referred to as a specified P-E block copolymer), obtained by polymerizing in the first polymerization stage to give 70-95% by weight of propylene homopolymer (based on the entire copolymer) having a relation between isotactic pentad ratio (P) and melt flow rate (MFR) of 1.00.gtoreq.P.gtoreq.0.015 log MFR+0.955, and then polymerizing in the subsequent one or more stage to give 30-5% by weight of propylene-ethylene copolymer (based on the entire copolymer), or
modified propylene-ethylene block copolymer formed by modifying the specified P-E block copolymer by an unsaturated carboxylic acid or its derivative (hereinafter referred to as specified modified P-E block copolymer), or
a composition formed by blending a predetermined amount of organic fibers and an inorganic filler with a specified P-E block copolymer, a specified modified P-E block copolymer, or a mixture of the specified P-E block copolymer and the specified modified P-E block copolymer, has good moldability, and provides moulded articles having good rigidity, resistances to heat distortion to secondary deformation, to sink mark forming and to warpage deformation when it is made into moulded articles and we have completed the present invention based on these findings.
As evident from the foregoing description, the object of the present invention is to provide a polypropylene resin composition having a good moldability, which can produce moulded articles superior in rigidity, resistances to heat-distortion, to secondary deformation, to sink mark forming, and to warpage deformation when it is made into moulded articles.